1. Field of the Invention
The present invention relates to a method for conditioning metal powder to provide an improved feedstock for injection molding.
2. The State of the Art
There are a variety methods of forming parts from commonly used engineering materials. The artisan's choice of a desired processing method is often constrained by the material of which the part is composed and the final geometry of the part. Thus, one may take a block of material and machine it to the desired shape within the design tolerances, but environmental considerations (such as the dust generated) and tool wear caused by machining often make such processing uneconomical.
Recent advances in pure and applied sciences have created a need for high tolerance parts in relatively complex shapes; that is, shapes other than the common block, rod, disk, billet, and such shapes common for raw or semi-finished materials. For engineering plastics, this need has been fulfilled for some applications by injection molding, which is a conventional plastics processing technology. Injecting a polymer solution or melt into a closed mold affords the production of a final piece having the geometry of the mold. Mold making arts, especially for plastics injection, have advanced to providing molds with very high dimensional tolerances. While polymer compositions may shrink or even expand upon curing, thus requiring the mold designer to compensate for the volume change, the advantages of injection molding come from the ability of the injected fluid composition to completely fill the mold and thereby assume a complex geometry. If the mold is designed accurately and is completely filled by the injected composition, then the as-molded part is expected to have a high dimensional tolerance and very little machining may be necessary to yield the final part.
The injection molding of plastics (i.e., polymeric compositions) is facilitated by the ability of such compositions to flow. As alluded to above, a polymer may be dissolved in a solvent, injection molded, and the solvent driven off by heating; the polymer may be melted and then injection molded; or a prepolymer or monomeric composition may be injection molded with a catalyst to promote curing in the mold. In any case, the advantages of injection molding of plastics are afforded by providing a pourable, pumpable, or otherwise flowable composition suitable for injection molding.
Recent developments in such arts as electronics and engine technology have created a need for complex parts comprised of inorganic materials such as metals and ceramics. Some metal parts of complex shapes may be fabricated by stamping them out of a sheet; however, this process is wasteful (not all of the sheet is used, and the rest may not be able to be recycled economically), may not provide sufficiently high tolerances, and can create stresses within the part stamped from the sheet.
More recent advances in the arts of injection molding have been applied to metals (often termed "metal injection molding," MIM, or "powder metal molding," PMM). In general, injection molding of metal powders involves first mixing or "batching" the powder with a carrier vehicle and/or a binder, and then injection molding the batched powder to produce a "green" article. This green article typically is first processed to remove any remaining organic constituents and then densified or "sintered" to produce a final metal article. There are other techniques for forming complex parts from metals. A metal may be cold- and/or hot-worked to arrive at the desired product.
One of the primary considerations for any injection molding process, whether of a plastic or metallic composition, is the viscosity of the injected composition. Whether the initial composition is a polymer solution or a dispersion of metal particles in a fluid (a "slurry"), energy is required to pump the injected composition into the mold. The ease with which the compostion flows into the mold will have numerous effects on the part itself, the molding apparatus, and the economics of the process. A more viscous slurry will require more energy to be injected into the mold, and thus more expensive apparatus is needed both to create the high injection pressures and to keep the mold closed (to prevent the slurry from leaking out under this high injection pressure). A more viscous slurry also requires a high injection pressure to assure that the mold is completely filled with the composition. Still further, a viscous slurry may fold over onto itself (similar to a thick syrup) during injection into the mold; these folds can trap gas bubbles, resulting in porosity and/or other defects in the final article. Slurries of inorganic particles such as metals are also quite abrasive, and higher injection pressures result in even more abrasion in the flow channel and in the mold; this leads to increased maintenance costs due to the more frequent replacement of very expensive items such as high tolerance molds.
When making a slurry, the art may describe the fluid with which the powder is mixed as a vehicle, a solvent, a binder, or any number of similar terms, often dependent upon the nature of the injection molding process. For any fluid system, the viscosity is effected by a myriad of variables. For example, the viscosity of a slurry composed of metal powder and a fluid vehicle will be effected by the characteristics of the solids (metal powder), the liquid (vehicle), and their interrelationship.
More particularly, the average particle size, the particle size distribution, and the shape of the metal particles will effect the viscosity. Very small particles will typically result in a more viscous slurry than with the same volume fraction of larger particles. Particles of an essentially uniform size typically will result in a lower viscosity slurry than particles having a high aspect ratio; "aspect ratio" is generally defined as the ratio of the particle length to its width or diameter, so a spherical particle has a low aspect ratio (L.apprxeq.D) and a whisker or fiber-like particle has a high aspect ratio (L&gt;&gt;D). The fluid itself, without having any particles dispersed in it, will also have some inherent viscosity. The viscosity of the fluid will increase as the volume fraction of the metal powder particles dispersed increases. Still further, the characteristics of the metal powder particles may promote or inhibit the formation of microscopic structures in the fluid, thereby leading to changes in the viscosity depending upon the shear rate, and possibly also a time-dependent shear-viscosity relationship.
The production of a flowable, pumpable, or injectable mixture of powder and vehicle advantageously allows production of parts by injection molding. However, the art of dispersing solids in a fluid or fluidizable vehicle depends primarily upon empirical experimentation to determine useable and optimal systems. The fluidity (including pourability, pumpability, and other rheological aspects) of the feedstock generally depends upon various characteristics of both the solid and liquid phases. One of the most conventional methods for altering the viscosity of a slurry, although still determined empirically, is by the use of one or more dispersants. This determination is empirical because of the myriad interactions in the liquid-solid system and thus some dispersants may lower viscosity while others may increase viscosity; also, a suitable dispersant for use at a design shear rate may not be suitable at a different shear rate.
A disclosure in the art of MIM by Bernard Williams ("Cost Effective Production of Fine Metal Powders by Fluidised Bed Jet Milling," Metal Powder Report, Shrewsbury, UK, Jan. 1989, pp. 38-40) only teaches that "fine" metal powders having an average particle size of less than 20 .mu.m are suitable for injection molding. Williams also describes that to achieve this criterion, the optimum starting material for fluidized bed jet milling should have an average particle size of less than 2 mm, preferably 50-300 .mu.m. Without disclosing the actual characteristics of the starting powders, Williams describes product powders of various compositions having average particle sizes ranging from 0.9 .mu.m to 14 .mu.m, with 97% of the product having sizes ranging from less than 2 .mu.m to less than 36 .mu.m. This article also lacks any quantitive assessment by which the actual suitability for injection molding the product powders could be judged.
Given these and other parameters that effect the rheology of a solids dispersion, and the empirical nature by which workable vechicle-solid systems are devised, it is not readily predictable which systems will result in useable, injectable feedstocks.